1. Field of the Invention
The present invention relates to a fuel cell system that is equipped with a fuel gas circulating system.
2. Description of the Related Art
Conventional fuel cell systems generally include a fuel cell, which is supplied with a fuel gas that contains hydrogen and an oxidation gas that contains oxygen, to generate electric power and a means to prevent freezing in a low-temperature environment. For example, in Japanese Patent Application Publication No. 2003-203665 (JP-A-2003-203665), dry air is fed into a flow rate control valve provided in an oxidation gas supply system to blow off water droplets thereon when the fuel cell is stopped.
A conventional fuel cell system may also be provided with a discharge valve in a circulating system, which is used to circulate and supply fuel off gas to a fuel cell with a circulation pump. By opening the discharge valve as needed when the fuel cell system is operating, impurities (nitrogen gas and so on) contained in the fuel off gas may be discharged from the circulating system together with the fuel off gas and the hydrogen concentration in the circulating system is thereby prevented from decreasing. Also, the water (vapor) that is generated through the electrochemical reaction in the fuel cell is actively discharged into the circulating system together with the fuel off gas by the circulation pump and then out of the circulating system through the discharge valve.
In a low-temperature environment below 0° C., if water discharged from the fuel cell into the circulating system adheres to the discharge valve or the inner surfaces of the flow path from the fuel cell to the discharge valve, the water may freeze and cause an opening failure of the discharge valve. In particular, an opening failure of the discharge valve that is caused by freezing of the discharged water tends to occur during startup of the fuel cell because the amount of heat in the generated water is small. Providing a heater that is used to melt frozen parts as a measure against such freezing leads to an increase of equipment and cost for the entire fuel cell system. The measure that is described in JP-A-2003-203665 is not necessarily effective because it does not relate to the time of startup of a fuel cell, when freezing is most likely to occur.